1. Field of the Invention
The present invention relates to air pumps and, more particularly, to a noise reduction device for use in air pumps.
2. Description of the Related Art
An air pump sucks in air from the surroundings, compresses the air and discharges the compressed air. In this regard, vibration noise generated in a drive unit of the pump is transmitted to the outside through an air suction passage and so forth, thus emitting noise to the surroundings. Accordingly, various noise reduction devices have been developed.
For example, the following patent literature 1 discloses a noise reduction device having a circular cylindrical housing accommodating an air pump unit, wherein the housing is provided with noise reduction means. In the noise reduction device, an inner wall is provided inside a circular cylindrical wall (outer wall) of the housing to extend over about 300 degrees in parallel to the cylindrical wall to form a suction passage between the two walls. In one end of the passage, the inner wall is connected to the cylindrical wall to form a closed end. The other end of the passage is an open end. Air outside the housing is sucked into the suction passage from near the closed end and passed through the passage to the open end, from which the air enters the interior of the housing, in which the pump unit is accommodated. In the noise reduction device, the suction passage is lengthened in this way to suppress noise from being transmitted to the outside through the suction passage.    Patent Literature 1: Japanese Examined Utility Model Application Publication No. 1994-00627